This invention relates to enhanced oil recovery. More specifically, this invention relates to displacing oil within a subterranean reservoir by injecting into the reservoir steam and an improved steam foam-forming surfactant which rapidly propagates in the reservoir to form a strong steam foam. Many hydrocarbons are too viscous to be recovered from subterranean oil-bearing formations without assistance. These heavy hydrocarbons can be recovered through the use of steam drives which heat the formation, lower the viscosity of the hydrocarbons, and enhance the flow of the hydrocarbons toward a production well. However, after initial injection breakthrough at the production well, the injected steam preferentially follows the breakthrough path. Also, except near the injection and production wells, the active steam zone in the reservoir tends to rise to the upper levels of the oil-bearing formation. Thus, the total amount of the formation that is swept by the steam injection is limited.
Surfactants have been injected along with steam to create a steam foam flood. The combination of steam and steam foam surfactant results in a steam foam, which is a dispersion of steam vapor in a continuous water phase, wherein at least part of the steam vapor phase is made discontinuous by liquid films, or lamellae. The presence of this foam is indicated by a reduction in the rate at which the steam travels through the reservoir. The foam creates a barrier that slows movement of the steam both to the upper levels of the oil-bearing formation, and towards the production well. This mobility reduction results in more efficient heat transfer to the oil, which increases oil recovery at the production well and results in lower average residual oil saturation in the reservoir when the steam foam drive is completed.
To maximize the rate at which oil is recovered, it is desirable to have a steam foam surfactant which forms foam rapidly in the reservoir. An efficient transport of the surfactant through the reservoir is important, because the rate at which the foam moves through the reservoir is limited by the rate at which the surfactant moves through the reservoir. Transport of the surfactant through the reservoir may be reduced by a variety of mechanisms, such as adsorption on the reservoir rock, precipitation by divalent ions present in the formation water and/or ion-exchanged off of the reservoir rock, and partitioning into the oil phase. Sufficient quantities of surfactant must be moved quickly through the reservoir to sustain the foam in a steam foam drive operation. It may be assumed that the foam transport rate Is comparable to the surfactant transport rate.
Numerous prior processes have involved various uses of steam in conjunction with a surfactant, and improvements to such processes. U.S. Pat. No. 3,292,702 suggests a steam soak process in which an aqueous surfactant is injected ahead of the steam to provide an increased injectivity during the steaming period and a greater rate of production during backflow. U.S. Pat. No. 3,357,487 reveals injecting a solution of surfactant prior to or during a steam injection so that a band of the surfactant solution is displaced by the steam. U.S. Pat. No. 3,412,793 suggests that, in a relatively highly stratified reservoir, a steam soak or steam drive process for recovering oil is improved by temporarily plugging the more permeable strata with foam. U.S. Pat. No. 4,086,964 discloses recovering oil by injecting a steam foam-forming mixture through a steam channel which extends essentially between injection and production wells. U.S. Pat. Nos. 4,393,937 and 4,488,976 describe a steam foam-forming mixture in which the surfactant is a particularly effective alpha olefin sulfonate as well as methods of using such a mixture in steam drive or steam soak oil recovery processes. U.S. Pat. No. 4,488,598 discloses a steam and gas distillation drive using a foamable surfactant. U.S. Pat. No. 4,597,442 suggests a preflushing solution for increasing the rate at which the injected surfactant is propagated through the reservoir by reducing ion-exchange effects. U.S. Pat. Nos. 4,556,107 and 4,607,700 disclose a steam foam injection process improved by the use of alpha olefin sulfonate dimer surfactants. U.S. Pat. No. 4,609,044 describes an alkali-enhanced steam foam drive or soak process for recovering low gravity acidic oil. U.S. Pat. No. 4,617,995 discloses injection of a pretreating fluid ahead of at least some of the steam and steam foaming surfactant to increase the rate of surfactant transport and decrease the amount of surfactant required. U.S. Pat. No. 4,643,256 suggests a steam foaming surfactant mixture which is effective even in the presence of multivalent cations.